BOBBY Kennedy Jr. is his father’s son in this respect: Politics – cut-throat, down-and-dirty Democratic politics – are in the marrow of his bones.

So excuse him for thinking he should help derail Rudy Giuliani’s presumed bid for the U.S. Senate. A Kennedy’s gotta do what a Kennedy’s gotta do.

But don’t forgive his effort to politicize the New York watershed. He’s playing with fire – but it’s the city that stands to get burned.

Two years ago, Kennedy was one of the principal players in a compact between groups so profoundly suspicious of one another that negotiations – let alone an agreement – was nothing short of astonishing.

So far, anyway, it was his finest moment.

The issue was water – cool, clear water from the city’s upstate reservoir system. The question was this: Must New York City spend a minimum of $8 billion – and probably a lot more – on a water-filtration system that will cost more than $500 million a year to operate?

There is very little wiggle room here. The federal Environmental Protection Agency, under the Clean Water Act, has the power simply to order filtration. And it was – and probably remains – very close to doing just that.

If it does, the costs will be passed directly to property owners. In much of the city – and, in particular – residential Queens and Brooklyn – this would mean hugely inflated water-tax bills for single-family homeowners.

Which is bad enough.

Worse, the impact on owners of multi-unit rental properties stands to be even more substantial. As a practical matter, it is next to impossible to meter individual apartments – so the full burden of the new water taxes would fall on landlords who, because of rent-control laws, would find it very difficult to pass them along to tenants.

The last time landlords had to swallow sharp hikes in fixed costs – during the inflation-driven ’70s – a lot of marginal properties burned in the night. Whole neighborhoods burned in the night.

And, frankly, the prospect of another arson epidemic can’t be at the top of the city’s watershed worry list.

New York cannot afford to suck $8-plus billion out of the funds available for capital reconstruction. A dollar spent on filtration is a dollar unavailable for school repairs, highway and bridge reconstruction or low-income housing.

This is why the 1997 watershed pact was so huge. Upstate townships agreed to greatly limit development where it would threaten the city’s supply of clean water. In exchange, the city – with an obvious interest in pure water, and with an EPA gun to its head – agreed to pay the communities a cool $1.5 billion. It’s a bribe; everything else is detail.

Now the question now is this: Are all parties sufficiently respectful of those details?

Kennedy, who recently discussed the matter with Hillary Rodham Clinton, says no. He says, in fact, that “Rudolph Giuliani is trading the watershed for upstate votes.”

But here’s what Ed Heelan, a Westchester County real-estate guy with obvious interests in play, said yesterday when told of Kennedy’s allegation:

Long pause.

“Ha!”

Says Heelan: “That’s totally wrong. [Bobby Kennedy] is fanatical. He’s trying to shut down development altogether. And so is the city.

“That’s why we’re in court.”

As, indeed, he is.

Heelan is the lead plaintiff on a class-action suit that’s seeking mega-billions in damages for what he terms the loss of local property rights under the watershed agreement.

Kennedy says that he has filed a suit of his own – and it’s hard not to suspect that many young lawyers will someday be sending their kids to college on fees from continuing watershed litigation.

But New York City doesn’t have that kind of time. Right now, it’s well into the third year of a five-year stay of an EPA order to begin construction of a filtration system. And you can bet that the EPA bureaucrats are itching to issue that diktat – because that’s what bureaucrats live for.

What’s needed right now – the exigencies of Hillary’s campaign notwithstanding – is an extra-large serving of good will.

That, and everybody coming to understand that time is on upstate’s side. Once filtration has been ordered, there will be scant reason to worry about pollution – and it will become much easier to build upstate. They could pave paradise, such as it is, and put up a parking lot.

But, for now, Heelan says he wants to talk: “We’re suing for billions, but we just want to get the city to the table in a reasonable frame of mind.”

One thing that will get everybody in an unreasonable frame of mind – real quick – is the injection of partisan politics into this debate.

Bobby Kennedy, with his intemperate assertions, is tying a big rock around the city’s neck. Hillary Clinton doubtless doesn’t know any better – or even care. Surely, Kennedy should. —